The present invention relates to a video signal encoding/decoding method, and more particularly, to a prediction method for a discrete cosine transform coefficient, for predicting DC/AC coefficients of a current block using location information of nonzero AC coefficients in a discrete cosine transform (DCT) domain.
A discrete cosine encoder encodes without loss of DC/AC coefficients obtained after quantization. Here, the DC/AC coefficients of the current block are predicted using the fact that adjacent blocks are similar to each other.
FIG. 1 is a diagram illustrating a general DC/AC coefficient prediction method. In order to predict the coefficients of the current block 101 which has been quantized after the DCT, coefficients of adjacent blocks 102 and 103 which are obtained through the same process are used. DC coefficient 104 of the current block 101 is predicted using the DC coefficient 107 of an upper block 102 or the DC coefficient 109 of a left block 103. This is applied only for the prediction of the DC coefficient. An AC coefficient area 105 having the vertical characteristics of the current block 101 to be coded is predicted from the AC coefficients of an area 108 of the upper block 102. This prediction method is profitable when the vertical components between blocks of an image are similar. An AC coefficient area 106 having the horizontal characteristics is predicted from the AC coefficients of an area 110 of the left block 103. This prediction method is profitable when the horizontal components between blocks of the image are similar. The prediction of the DC/AC coefficients of the current block 101 is performed in the areas 104, 105 and 106.
FIG. 2 illustrates a general lossless encoding method of quantized coeficients in a DCT coding. In the block, a run-length coding is performed by zigzag scanning 201.
FIG. 3 shows the structure of an encoder/decoder of DCT coefficients using a general DC/AC coefficient prediction method. Data 301 is converted into a DCT coefficient 303 by a discrete cosine transformer (DCT) 302. The DCT coefficient 303 is quantized by a quantizer 304. A quantized DCT coefficient 305 is input to a DC/AC coefficient predictor 306 for the prediction. Predicted coefficient 307 is encoded by a variable length code (VLC) encoder 308 and output as a bit stream 309. The bit stream 309 is decoded by a VLC decoder 310. Decoded coefficient 311 is compensated by a DC/AC coefficient compensator 312, resulting in a compensated quantized coefficient 313. The compensated quantized coefficient 313 is inversely quantized by an inverse quantizer 314, and the obtained DCT coefficient 315 is inversely transformed by an inverse DCT (IDCT) 316, resulting in final restored data 317.
As described above, according to the general DC/AC coefficient prediction method, DC and AC coefficients in a horizontal or vertical area are predicted from the corresponding area of an adjacent block as shown in FIG. 1. When performing a DCT on a block, a DC coefficient corresponds to the brightness of the block, which is similar among adjacent blocks. Also, in the case of the AC coefficient, the vertical and horizontal components of the block are reflected in AC coefficients in a DCT domain, so that the prediction can be achieved if the vertical and horizontal components of each adjacent block have similarity.
However, the general DC/AC coefficient prediction method uses the results of the DCT of the adjacent blocks regardless of their value, so that the improvement in the prediction performance is less than optimum. Also, when predicting an AC coefficient having a value of zero, there is a high probability that the predicted AC coefficient is not equal to zero, thus the prediction performance can not improve. A new algorithm is required for solving these problems.